Ever more stringent emissions regulations have driven the compression ignition engine industry to adopt increased fuel injection pressures. One area of concern as a consequence of increased injection pressures relates to potential fatigue in the sac region of the fuel injector nozzle tip component. The sac region is often the thinnest pressure containment metallic layer, and also defines the nozzle outlets that extend between an interior volume of the fuel injector to the combustion space of the engine. The sac region will typically cycle through extreme high pressures with each engine cycle.
One strategy believed to have promise in strengthening fuel system components is to induce compressive residual stress on the inner surface the component. While a number of different strategies are possible for inducing compressive residual stress, an autofrettage process can be effective in inducing compressive residual stress on the interior surfaces of pressure vessels. An Autofrettage process typically involves subjecting an inside of the vessel to a large fluid pressure so that residual compressive stresses are introduced on the inside of the vessel. To accomplish autofrettage in a high pressure vessel having one or more orifices, the orifices may be sealed prior to application of fluid pressure on the inside of the vessel. U.S. Publication No. 2011/0126393 relates to a production robust autofrettage process for strengthening fuel injector nozzle tips. The process includes applying a vacuum to the interior volume of the nozzle tip. Plugs are suctioned over each of the nozzle outlets. Nozzle outlets are blocked by pressing the plugs between the nozzle tip and a fixture component. The nozzle tip is then autofrettaged at least in part by pressurizing the interior volume with an autofrettage liquid. The plugs are then removed from the nozzle outlets leaving the nozzle tip strengthened with compressive residual stress.